Animated ink-blot images keep unwanted bots at bay

THREE animals lurk in the image to the right. Your ability to pick them out might just be proof that you are a living, breathing human.

Such proof is highly prized online, where website operators strive to distinguish between human visitors and unwelcome software bots. That distinction has so far been made using distorted patterns of letters, called captchas, which are legible to humans but indecipherable to most bots. As bots have become more sophisticated, though, captchas have become more fiendish, making them tricky for even the most gimlet-eyed human, says Niloy Mitra at the Indian Institute of Technology Delhi.

Working with Daniel Cohen-Or at Tel Aviv University, Israel, and colleagues, Mitra reckons he has found a better alternative. The team created an algorithm to turn images into inkblots. Parts of ...

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